Ensure URL in triage mode is a proper URL

[jcscottiii]

Taking a look at this Github Action, you see that it breaks the build because the URL cannot be parsed because it is missing the protocol.

--- FAIL: TestParseMetadata/../html/semantics/popovers/META.yml (0.01s)
        parse_test.go:58: 
            	Error Trace:	/home/runner/work/wpt-metadata/wpt-metadata/go_test/parse_test.go:58
            	Error:      	Expected nil, but got: &url.Error{Op:"parse", URL:"crbug.com/325[9](https://github.com/web-platform-tests/wpt-metadata/actions/runs/8511859872/job/23312301217?pr=6215#step:5:10)81758", Err:(*errors.errorString)(0xc0003fbd[10](https://github.com/web-platform-tests/wpt-metadata/actions/runs/8511859872/job/23312301217?pr=6215#step:5:11))}
            	Test:       	TestParseMetadata/../html/semantics/popovers/META.yml

We should enforce on the client and server side that the URL is in proper format so that on call engineers do not need to make commits like this

Happened in two separate PRs:

• https://github.com/web-platform-tests/wpt-metadata/pull/6215
• https://github.com/web-platform-tests/wpt-metadata/pull/6216